The long-term objective of this research program is to examine the development of phonological categories by children with phonological disorders. The specific aims are to characterize the mental representation of phonological categories and to evaluate processing strategies used in categorization by this population. These goals will be accomplished by experimentally testing the course, types, principles, and limits of restructuring in phonological categories by these children. An interdisciplinary research approach is adopted, integrating theories and methodologies of cognitive psychology, linguistics, and speech-language pathology in the study of 72 children' Four main projects are planned, each involving phonological and acoustic descriptions of children's sound systems, assessments of children's judgments of phonological categories, and experimental clinical treatment. Project I examines the course of change in categories by evaluating the hypothesis that certain phonological categories are prerequisite for others in development. Project II differentiates types of change in categories by testing the hypothesis that some phonological changes are easier to induce clinically than others. Project HI takes up the issue of principles governing categories by examining the hypothesis that the degree and extent of clinically-induced change is influenced by a child's predetermined assumptions about the phonological system. Project IV tests the limits of category change by identifying children at risk for no phonological change. The outcome of this research program will contribute to an understanding of phonological categories, development, and change. There is strong applied significance in that the goal of clinical treatment is to induce such change. The findings to emerge will lead to treatment advances for facilitating the greatest change in sound systems. Theoretically, an integrated model of the development of categories as instantiated in the domain of phonology will result. Domain-specific properties of categorization, particular to phonology, will be distinguished from those that are attributable more generally to the cognitive capabilities of learners.